[Bug target/49001] New: GCC uses VMOVAPS/PD AVX instructions to access stack variables that are not 32-byte aligned

[npozar at quick dot cz]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49001

           Summary: GCC uses VMOVAPS/PD AVX instructions to access stack
                    variables that are not 32-byte aligned
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: target
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: npo...@quick.cz


I'm using a custom mingw64 build of GCC 4.6.1. My target is Windows 64bit. I
compile with g++ -03 -march=corei7-avx -mtune=corei7-avx -mavx.

GCC uses aligned moves VMOVAPS/PD from the new AVX instruction set to access
local variables of type __m256/__m256d on the stack. But the stack pointer is
only 16byte aligned on Win64, so this causes a segmentation fault error when
the stack pointer is not 32byte aligned, as in:

__m256 dummy_ps256;
void test_stackalign32() {
    __m256 x = dummy_ps256;
    dummy_ps256 = sin256_ps_avx(x);
}

which compiles to 

    vmovaps    dummy_ps256(%rip), %ymm0
    leaq    32(%rsp), %rdx
    vmovaps    %ymm0, 32(%rsp)  // possible SEGFAULT
    leaq    64(%rsp), %rcx
    vzeroupper
    call    _Z13sin256_ps_avxDv8_f
    vmovaps    64(%rsp), %ymm0  // possible SEGFAULT

I couldn't figure out how to realign a stack with -mstackrealign.